1. Field of the Invention
This invention relates to a molded surface fastener in which a multiplicity of hooks are molded on a substrate sheet by extrusion or injection molding of thermoplastic synthetic resin, and more particularly to a hook structure in which hooks to be molded of the same quantity of resin are improved in engaging strength and durability.
2. Description of the Related Art
Surface fasteners of the type in which hooks are formed by weaving monofilaments in a woven cloth so as to form loop piles of monofilaments and then cutting the loop piles are well known in the art. This type surface fastener has softness of a woven cloth and softness of monofilament and is characterized in that the hooked surface fastener comes into engagement with and are peeled off loops of a companion surface fastener with a very smooth touch. Moreover, since the monofilaments constituting the hooks are treated by drawing, the surface fastener is excellent in pulling strength and bending strength even in a small cross-sectional area. Further, since the surface fastener can have a very high density of hooks depending on the woven structure, it is possible to secure a high engaging rate and an adequate degree of durability. However, with the woven type surface fastener, since consumption of material and a number of processing steps are large, it is difficult to reduce the cost of production.
For an improvement, a molded type surface fastener was developed in which a substrate sheet and hooks are formed integrally and simultaneously by extrusion or injection molding. Typical examples of molding technology for this type surface fastener are disclosed in, for example, U.K. Patent No. 1319511 and WO 87/06522. As a rotary drum in which a number of molding disks each having on an outer peripheral edge of each of opposite surfaces a number of hook-forming cavities and a number of spacer disks each having flat surfaces are alternately superimposed one over another is rotated, molten synthetic resin material is forced against its peripheral surface to fill the cavities and then the hooks formed in the cavities are removed off the drum along with the substrate sheet. The spacer disks are disposed between the molding disks because the cavities of the whole shape of the hooks cannot be made in one mold due to the shape of the hooks.
However, in the molded type surface fastener, partly since a delicate shape cannot be obtained as compared to the woven type surface fastener due to technical difficulty in molding process, and partly since the formed hooks are poor in orientation of molecules, only a very low degree of strength can be achieved with the same size of the above-mentioned monofilament hooks. Therefore none of the conventional molded type surface fasteners are satisfactory for practical use. Further, according to the conventional hook structure, the individual stem is simple in cross-sectional shape and would hence tend to fall flat from its base. As a result, the individual stems would not restore their original posture after repeated use, thus lowering the rate of engagement with loops of a companion surface fastener. Therefore, in order to secure desired strength, it is absolutely necessary to increase the size of the individual hooks, which makes the hooks rigid and the number of hooks per unit area (density of hooks) reduced to lower the rate of engagement with the companion loops.
As a solution, a new hook structure which enables a smooth touch, with the stem hardly falling flat, during the engaging and peeling operation likewise the woven type surface fastener and which increases the rate of engagement to secure adequate strength is disclosed in, for example, U.S. Pat. No. 5,131,119. In the molded type surface fastener disclosed in this U.S. Patent, each hook has a hook-shape engaging portion extending forwardly from the distal end of a stem which has a rear surface rising obliquely in a smooth curve from a substrate sheet and a front surface rising upwardly from the substrate sheet, and a reinforcing rib projecting from a side surface of the stem, the cross-sectional area of the hook increasing gradually from a tip of the hook-shape engaging portion toward the base of the stem. The reinforcing rib serves to prevent the stem from falling laterally and also to minimize the size of the stem and the hook-shape engaging portion, maintaining a required degree of engaging strength to the stem and the hook-shape engaging portion.
According to the conventional molded hook structure, it is totally silent about the transverse cross-sectional shape. Also in the above-mentioned prior art references, the respective molded hook structure has merely a triangular, a rectangular or a circular (including an oval) transverse cross-sectional shape. Therefore in the transverse cross-sectional shape taken along a plane perpendicular to the axis (center line) of the hook, the cross-sectional area is divided into front and rear cross-sectional areas with respect to the center line, and the rear side cross-sectional area is set to be equal to or larger than the front side cross-sectional area in either the stem or the hook-shape engaging portion. This means that the center of figure is located on the center line or the rear side of the hook.
When the molded hook is disengaged from the loop of the companion surface fastener, a tensile stress occurs inside the front part of the hook with respect to its neutral line while a compressive stress occurs inside the rear part of the hook. In general, this type hook of synthetic resin is resistant against a compressive stress but is remarkably less resistant to a tensile stress compared to a hook of rigid material. Accordingly, in the case of the conventional cross-sectional shape, small hooks in particular are not only too low in strength but also high in flexibility, so that the force of engagement with loops is remarkably lowered. When hooks having large transverse cross-sectional area are disengaged from loops, they would tend to be broken or damaged as the tensile stress in the front part of the hook increases according to the magnitude of the engaging force.